pricing my brooklyn property

[Ryan H.]

I am not sure how to price the building that I want to list in borough park-kensington area.  The lot is 20x100 building is 20x70 2 stories and basement. 1st floor store front and basement (currently rented at 3500 should be 5500plus comps show) 2nd floor a 1bd + 2bd apt (rent roll is 2500 under market should be 3500+). Taxes are 5g. All spaces have no lease can be delivered vacant. So at those calculations I come up with at 2.5m its cap is 4.5%.  That is not including the value of it being zoned R6 and can go up 2 more stories like every other building in neighborhood has.  The question that I am wondering is the what is the value if any added for future add on development for the 2 stories?  With not many property comps available at moment with area I don't want to short change myself or overprice I need gone asap. Thanks for your time